The present application claims priority to Japanese Application No. P2000-034376 filed Feb. 7, 2000, which application is incorporated herein by reference to the extent permitted by law.
1. Technical Field
The present invention relates to a magneto-optical recording medium for writing or reading information by means of magneto-optical recording. More particularly, the present invention relates to a magneto-optical recording medium having increased recording density by means of the magnetically induced super resolution technology.
2. Prior Art
A magneto-optical recording medium is practically used as a rewritable recording medium for writing or reading information according to a magneto-optical recording technology.
In recent years, particular attention is paid to a technology called the magnetically induced super resolution (MSR) in the field of magneto-optical recording media for increasing recording density. This technology makes it possible to read information from a recording mark which is smaller than a spot diameter of a light beam used for recording and reproducing information.
The MSR uses temperature distribution in a spot of the light beam for recording and reproducing information and masks a low- or high-temperature region in the spot. This enables to read information from a recording mark which is smaller than a spot diameter of the light beam.
As an example of the MSR, Jpn. Pat. Appln. Laid-Open Publication No. 5-81717 discloses a system called CAD (Center Aperture Detection).
The CAD system uses-a magneto-optical recording medium comprising a read layer, a recording layer, and a non-magnetic film. At room temperature, the read layer maintains an in-plane magnetization state in which in-plane magnetic anisotropy is predominant. As a temperature rises, the read layer becomes a vertical magnetization state in which vertical magnetic anisotropy is predominant. The recording layer records and stores information. The non-magnetic film is placed between the read layer and the recording layer for preventing both layers against exchange coupling. Information is recorded on the recording layer of the magneto-optical recording medium through the use of a recording mark which is smaller than a spot diameter of the light beam used for recording and reproduction.
When a light beam is irradiated onto the read layer during reproduction, the laser beam heats the read layer and increases its temperature. At this time, a light beam is stopped down to the diffraction limit by means of a condensing lens. The corresponding light intensity distribution becomes the Gaussian distribution. Temperature distribution on the read layer also becomes the Gaussian distribution. When a light beam is irradiated to a given region on the read layer, only the region""s center is heated to approximately a magnetic compensation temperature and becomes a vertical magnetization state.
A leakage field is applied from the recording layer to the region in the vertical magnetization state on the read layer, transferring a recording mark on the recording layer. Only the light beam spot center becomes the vertical magnetization state on the read layer. The other regions remain in the in-plane magnetization state. Accordingly, the other regions mask magnetization of the recording layer. As a result, it is possible to read information from a recording mark smaller than the light beam spot diameter.
The above-mentioned CAD system is a very excellent technology, providing advantages of having relatively simple film configuration and eliminating the need for applying an external magnetic field for initialization during reproduction.
However, the CAD system transfers recording marks on the read layer by using a leakage field from the recording layer. Therefore, the CAD system causes a problem of a decreased transfer force compared to the other systems which uses exchange coupling for transferring recording marks.
In the CAD system, a transfer force depends on recording mark lengths. Namely, a transfer force with a long recording mark is weaker than that with a short recording mark. Generally, a magneto-optical recording medium uses the recording mark length of 2T through 8T, where T is one channel clock cycle. The longest mark is four times longer than the shortest mark. Accordingly, even if a relatively short recording mark is transferred, a relatively long recording mark is not transferred properly, thus eventually causing a read error.
For increasing a transfer force of the recording mark, it is effective to change the composition of the recording layer comprising a rare-earth transition-metal (R-TM) alloy film so that the composition becomes TM-rich. Alternatively, it is effective to increase the saturation magnetization (Ms) for the recording layer by increasing the recording layer thickness, for example. Namely, a temperature (read temperature) is generated when the read layer changes from the in-plane magnetization state to the vertical magnetization state. When the saturation magnetization (Ms) is increased for the recording layer at the read temperature, it is possible to increase the transfer force of the recording mark by strengthening the leakage field from the recording layer.
However, if an attempt is made to simply increase the saturation magnetization (Ms) for the recording layer, this magnetization also increases in a temperature range lower than the read temperature. Consequently, the read temperature is spread to a low-temperature side, thus broadening a read area and degrading the spatial resolution.
The present invention has been made in consideration of the foregoing. It is therefore an object of the present invention to provide a magneto-optical recording medium which can increase a transfer force of the recording mark by strengthening a leakage field from the recording layer without degrading the spatial resolution.
After repeated examinations for solving the above-mentioned problems, the inventor found the following. Namely, when a recording layer for recording and storing information comprises a plurality of layers with different compositions, saturation magnetization for the recording layer can be higher than that for a single-layer recording layer at around a read temperature. Alternatively, the saturation magnetization can be lower than that for a single-layer recording layer in a temperature range lower than the read temperature. When such a recording layer is used for a magneto-optical recording medium, it is possible to increase a transfer force of a recording mark without degrading the spatial resolution.
A magneto-optical recording medium according to the present invention is originated on the basis of the above-mentioned knowledge and is characterized by comprising a read layer maintaining an in-plane magnetization state predominantly characterized by in-plane magnetic anisotropy at room temperature and changing to a vertical magnetization state predominantly characterized by vertical magnetic anisotropy as temperature rises, a recording layer for recording and storing information, and a non-magnetic film sandwiched between said read layer and said recording layer for preventing exchange coupling between said read layer and said recording layer, wherein said recording layer comprises a first layer and a second layer with different compositions from each other; said first layer is made from a recording material giving parallel directions to saturation magnetization and sublattice magnetization; and said second layer is made from a recording material giving antiparallel directions to saturation magnetization and sublattice magnetization at room temperature and giving parallel directions to saturation magnetization and sublattice magnetization at around a read temperature at which said read layer changes from an in-plane magnetization state to a vertical magnetization state.
A recording layer of this magneto-optical recording medium comprises a plurality of layers, namely first and second layers. The first layer comprises, say, an amorphous thin film or the like made from a TbFeCo alloy. Such a recording material gives parallel directions to saturation magnetization and sublattice magnetization on the recording layer. The second layer comprises, say, an amorphous thin film or the like made from a GdFeCo alloy. At room temperature, such a recording material gives antiparallel directions to saturation magnetization and sublattice magnetization. At around a read temperature, the material gives parallel directions to saturation magnetization and sublattice magnetization. The saturation magnetization for the recording layer is higher than that for a single-layer recording layer at around a read temperature. Alternatively, the saturation magnetization is lower than that for a single-layer recording layer in a temperature range lower than the read temperature.
The saturation magnetization for the recording layer increases at around a read temperature. Because of this, it is possible to increase a transfer force of the recording mark by strengthening a leakage field from the recording layer. The saturation magnetization for the recording layer decreases in a temperature range lower than the read temperature. This prevents a read area from expanding due to the read temperature shifting to a low-temperature side and maintains the spatial resolution.
A magneto-optical recording medium according to the present invention should maintain the relation Tc1 less than Tc2, where Tc1 is a Curie temperature of the recording material constituting the first layer of the recording layer; Tc2 is a Curie temperature of the recording material constituting the second layer of the recording layer. To achieve this relation, it is desirable to use the second layer""s recording material which provides a higher Curie temperature than the first layer""s recording material. When such a recording material is used for the second layer of the recording layer, it is possible to generate a leakage field from the recording layer also at around a Curie temperature on the first layer with relatively large saturation magnetization for the entire recording layer. A recording mark can be transferred properly even if the read area center is available at an approximate Curie temperature on the first layer.
For the magneto-optical recording medium according to the present invention, it is desirable to provide an auxiliary read layer comprising, say, a GdFe alloy between the read layer and the recording layer in order to drastically change a read layer state at the read temperature. For this purpose, the use of such an auxiliary read layer enables to provide higher density.
As mentioned above in detail, the magneto-optical recording medium according to the present invention includes the recording layer for recording and storing information. This recording layer comprises a plurality of layers, namely first and second layers. The first layer uses a recording material which gives parallel directions to saturation magnetization and sublattice magnetization on the recording layer. The second layer uses a recording material which gives antiparallel directions to saturation magnetization and sublattice magnetization at room temperature. This material for the second layer gives parallel directions to saturation magnetization and sublattice magnetization at around a read temperature. The saturation magnetization for the recording layer is higher than that for a single-layer recording layer at around a read temperature. Alternatively, the saturation magnetization is lower than that for a single-layer recording layer in a temperature range lower than the read temperature. Accordingly, this magneto-optical recording medium can increase a transfer force of a recording mark without degrading the spatial resolution.